This invention relates generally to a filter combining surface acoustic wave resonators and more particularly to the structure of a band-pass or band-rejection filter comprising the combination of a plurality of one-port surface acoustic wave resonators having combtooth-like electrodes or so-called interdigital finger electrodes disposed on a piezoelectric substrate.
Surface acoustic wave filters having various structures are known. (A. J. Slobodnik, Jr., T. L. Szabo, and K. R. Laker, "Miniature Surface-acoustic-wave filter," Proc. IEEE, vol. 67, P.129, 1979). However, the heretofore known filters convert all the input electric signals as such to acoustic signals (elastic surface waves) and then convert all the signal components as such to the electric signals. Therefore, signals of the pass band having considerable high energy are naturally converted to the acoustic waves. When the filter is applied for high power application, a surface acoustic wave with large amplitude propagates through the substrate surface. When the signal energy to be transmitted through the piezoelectric substrate surface is excessively high, mechanical strength of the interdigital finger electrodes becomes a problem. In other words, the electromechanical migration of metal electrodes (interdigital finger electrodes) due to high power affects transmission characteristics of the filter significantly.
To solve this problem, the inventors of the present invention proposed previously a filter combining surface acoustic wave resonators by connecting in series a plurality of one-port surface acoustic wave resonators between the input and output of the filter and connecting a capacitor between the terminal of each resonator and a common ground (Japanese Patent Laid-Open No. 220511/1986). The prior invention (which will be hereinafter called the "previous proposal" in order to distinguish it from the invention of the present application) has the following construction.
FIG. 1 shows an example of the filter combining surface acoustic wave resonators in accordance with the previous proposal.
In the drawing, reference numeral 1 designates a signal input terminal and 2 designates an output terminal of a filter. Common electrodes 91.about.96 are juxtaposed on a piezoelectric substrate 8 and finger electrodes are inserted alternately and connected to the common electrodes. The common electrode 91 constitutes an input terminal and a matching circuit consisting of inductances 3 and 4 with an input load is formed between it and the signal input terminal 1. The common electrode 96 constitutes an output terminal and inductances 5 and 6 forming a matching circuit with an output load is disposed between it and the output terminal 2 of the filter. Incidentally, the surface acoustic wave filter can be expressed by an equivalent circuit diagram of FIG. 2.
The parallel inductances 3, 6 and the series inductances 4, 5 connected to the input and output terminals 1, 2 represent external matching circuits analogous to those of FIG. 1.
Capacitances represented by the dotted line in the diagram represent the capacitances to the finger electrodes for each resonator and the electrodes connecting the resonators against the common ground or the capacitances against the common ground of the input and output terminal electrodes against the common ground. Though these capacitances are represented by dotted lines in FIG. 2, they can be set arbitrarily by increasing or decreasing the area of each electrode connecting the resonators or the area of the input and output terminal electrodes such as bonding pads. They can be set arbitrarily, too, by adjusting the thickness of the piezoelectric substrate 8. Furthermore, these capacitances may be formed by chip capacitors that are disposed outside, whenever, necessary.
According to the circuit configuration shown in FIG. 1, the impedance of the resonator is substantially the capacitance (30.about.33) between the electrodes alone in a frequency nearer to the resonance frequency, as can be understood from the equivalent circuit shown in FIG. 2, and can be expressed as shown in FIG. 3a. In the proximity of a resonance point, the resonator can be expressed in the form approximate to the series connection (301.about.304) of inductances and capacitances due to resonance as shown in FIG. 3b. In the proximity of a anti-resonance point, on the other hand, the resonator can be expressed in the form approximate to parallel connection (321--324) of the capacitances between the electrodes of the resonator and the inductances due to resonance as shown in FIG. 3C. At a frequency sufficiently higher than the anti-resonance point, the resonator becomes again the capacitances between the electrodes alone and can be expressed as shown in FIG. 3a. Incidentally, reference numerals 34.about.39, 311.about.316 and 331.about.336 represent the equivalent capacitances between the electrodes of the resonators and the common ground.
In such a circuit configuration, it is generally preferred that the pass band of the filter be set near the resonance frequency of the resonators. In this band, FIGS. 3a and 3b are simplified and can be expressed approximately as shown in FIG. 3d. In other words, since the surface acoustic wave resonators can be expressed by only the capacitance with respect to the common ground, matching with input and output loads can be attained always by external circuits.
At a frequency lower than the pass band, on the other hand, the influence of the series capacitances becomes relatively greater and the filter enters a stop band. At a frequency higher than the pass band or in other words, in the proximity of the anti-resonance frequency, the resonator can be expressed as shown in FIG. 3c and the impedance of the surface acoustic wave resonator becomes substantially high due to anti-resonance so that the filter enters the stop band. At a frequency sufficiently higher than the passing band, it can be simplified once again and expressed as shown in FIG. 3d but because the influence of the capacitance against the common ground becomes relatively large, the filter enters again the stop band.
The construction described above is suitable for a band-pass filter or band-rejection filter, for which extreme stop rise frequency characteristics are required on a higher frequency side of the pass band because the pass band is set near the resonance frequency and the stop band is set near the anti-resonance frequency.
Namely, the surface acoustic wave resonator according to the previous proposal made by the inventors of the present invention has excellent resistance to electric power and is extremely effective for accomplishing filter characteristics having a sharp rejection band on the higher frequency side of the pass band of the filter.
The inventors of the present invention examined the possibility of providing a filter having entirely opposite frequency allocation to that of the filter of the previous proposal, that is, a filter having the rejection band on the lower frequency side and the passing band on the higher frequency side, and also having sharp rise or fall characteristics.
As a result of intensive studies and production of prototypes, the inventors found out that the filter having the opposite frequency allocation as the first requisite can be implemented by delicately adjusting an external adjustment circuit inserted between the filter and the external load.
Though the filter can satisfy the first requisite, it has been found that if the second requisite, that is, the sharp rise or fall frequency characteristics, is to be attained, the loss of the passing band of the filter increases markedly and the filter has no practicality at all.
This is because the rejection band of the filter is formed by anti-resonance of each one-port surface acoustic wave filter constituting the surface acoustic wave resonator of the previous proposal. In other words, since the resonance point exists on the lower frequency side of the anti-resonance point in the one-port surface acoustic wave resonator, the loss of the filter in the pass band can be minimized by setting the pass band of the filter on the lower frequency side of the rejection band, that is, in the proximity of the resonance frequency. However, if the pass band is formed on the higher frequency side of the rejection band according to this structure as such, any contribution of resonance to the pass band can hardly be expected and the increase of loss in the pass band due to the external matching circuits (that is, by the influence of inductances in this case) cannot be neglected. This results in a serious problem particularly in those filters on which extremely severe loss characteristics are imposed, such as those filters which are used for mobile radio communication.